The invention concerns a device for propulsion control of an all-wheel drive vehicle. The device operates on the principle of keeping a vehicle wheel tending to spin within a range of values of propulsion slip compatible with both good propulsion acceleration and good driving stability by activating its wheel brake and, if all the vehicle wheels tend to spin, by initiating a torque reducing action in the drive unit of the vehicle.
A propulsion control device of this type is the subject of commonly assigned U.S. patent application Ser. No. 943,783 filed Dec. 19, 1986.
In this propulsion control device, the reference velocity v.sub.Ref (comparison of which with the wheel peripheral velocities v.sub.R of the vehicle wheels to be controlled makes it possible to recognize that propulsion slip thresholds have been exceeded) is determined from the wheel peripheral velocities of the front wheels of the vehicle. As long as these are not driven (in the case where the vehicle is provided with a permanent rear axle drive and a front axle drive which can be automatically switched on), their peripheral velocities can be used directly as the reference velocities for the left-hand and the right-hand sides of the vehicle. If the front wheel drive is switched on or if, in the case of a vehicle with permananet all-wheel drive, it is continually effective, the reference velocities are determined from low pass filtering by integration elements of the front wheel velocities. The filter time constants are continually changed as a function of the integration value of the vehicle longitudinal acceleration, which is itself in turn compared with the vehicle longitudinal acceleration, in such a way that the filter time constants are decreased with increasing vehicle longitudinal acceleration and the filter time constants are increased with decreasing vehicle longitudinal acceleration. The intention of this is that the reference velocity should follow the particular front wheel velocity considered "more rapidly" when the vehicle is powerfully accelerated than it does if the vehicle is only weakly accelerated when it must, in consequence, be assumed that the front wheels also enter a state of increased drive slip.
The propulsion control device in accordance with the older patent application operates quite satisfactorily if the vehicle is provided with an automatic all-wheel drive, with the meaning explained above, or with a permanent all-wheel drive which provides a fixed torque distribution between the rear and front axles in such a way that the major proportion of the drive torque is transmitted via the rear axle. In both cases, however, a complex drive train is necessary. Another disadvantage of this propulsion control device is that the reference velocity determined from the processing by integration of the front wheel velocities is only a very inaccurate measure of the vehicle velocity. Thus, in order to ensure sufficient driving stability, the slip thresholds at which the control is activated have to be set relatively low and consequently it is necessary to sacrifice a substantial amount of otherwise usable propulsion acceleration.
The objective of the invention is therefore to produce a propulsion control device of the type mentioned above which, without sacrificing driving stability, makes possible improved utilization of the motor output torque for propulsion acceleration and, including the drive train of the vehicle, can be effected with less technical complexity.
This objective is achieved in accordance with the invention by a propulsion control device for a motor vehicle which has a permanently balanced all-wheel drive, wherein the drive slip thresholds at which the control responds in the sense of a braking action, is increased as a function of the velocity and decreased as a function of the transverse acceleration. The acceleration thresholds, at which the control responds, are lowered as a function of the transverse acceleration. In the case of travel in a straight line, the lowest of the wheel peripheral velocities is taken as the reference velocity relative to which the drive slip thresholds are fixed. In the case of travel in a curve, the average of the wheel peripheral velocities of the left-hand side and right-hand side vehicle wheels is taken as the reference velocity for the vehicle wheels on the left-hand side and the right-hand side of the vehicle, respectively. From the time t.sub.0 at which the control at one of the vehicle wheels comes into operation, the velocity of this vehicle wheel is updated in accordance with the expression ##EQU1## and, at this value, is taken into account in the the formation of the reference velocities. The vehicle longitudinal acceleration b is continuously determined by means of a longitudinal acceleration sensor. In order to achieve optimum driving stability, additional control signals for the initiation of a torque reducing action in the drive unit of the vehicle are generated by analyzing the output signals of a transverse acceleration sensor, a curved travel sensor and the longitudinal acceleration sensor.
The subsequently provided manner of formation of the reference velocities up to the point where the control system responds, the updating of the wheel peripheral velocity after the control system responds, as the integral value of the vehicle longitudinal acceleration, the modification of the slip and threshold values as a function of the velocity and the transverse acceleration (the control system responding when these threshold values are exceeded) and the design of the drive train as a permanently balanced all-wheel drive achieves a propulsion control system using the simplest technical means. The control system provides optimum propulsion acceleration and, at the same time, guarantees adequate driving stability for the largest possible number of conceivable driving situations.
The slip thresholds, which determine when the control system responds and whose utilization supports driving stability, can be modified either alternatives or can be effected in combination as a function of velocity and/or curved path or transverse acceleration.
Similarly, the acceleration thresholds at which the control system responds can be modified as a function of the velocity or transverse acceleration.
A torque reducing action in the drive unit of the vehicle is initiated by the drive control device according to the invention whenever the control becomes simultaneously effective in the sense of a braking action on all the vehicle wheels, whether this occurs because the drive slip thresholds are exceeded or because the acceleration thresholds are exceeded.
Additional criteria for initiation of a torque reducing action in the drive unit of the motor vehicle, and whose observation gives a further improvement in driving stability, are provided. These include whenever and as long as the required value of a transverse acceleration a.sub.s determined from the vehicle velocity and the magnitude of the steering handle Lw is greater by a specified threshold value a.sub.s1 than the instantaneous value of the transverse acceleration a determined by the transverse acceleration sensor. During the course of travel in a curve, the second differential coefficient with respect to time a=.multidot.d.sup.2 a/dt.sup.2 of the instantaneous value transverse acceleration a is smaller than zero and is generated for at least as long as the signal characteristic of a&lt;0 is present. In association with these, are criteria which permit early reuse of good propulsion acceleration, without adversely affecting the driving stability. These include a device which terminates the control signal causing the torque reducing action in the drive unit if, after the signal characteristic of a&lt;0 has fallen off, the signal characteristic of a&lt;0 reappears. Also the device terminates the control signal causing the torque reducing action in the drive unit of the vehicle 12 if, after the control system comes into action, the acceleration value a (t.sub.0) occurring when it comes into action is again exceeded by the instantaneous transverse acceleration a acting on the vehicle.
Simple arrangements of logic circuits are provided, which individually or in combination can be used in the sense of the previously described criteria for the control of the torque-reducing action in the drive unit.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.